A field effect transistor such as a thin film transistor (TFT) is widely used as a unit electronic element of a semiconductor memory integrated circuit, a high-frequency signal amplification element, a liquid crystal driving element or the like. It is an electronic device which is most practically used recently.
Of these, with a remarkable development of displays in recent years, a TFT is widely used as a switching element which serves to drive a display by applying a driving voltage to a display device in various displays such as liquid crystal displays (LCD), electroluminescence displays (EL) and field emission displays (FED).
As the material for a semiconductor layer (channel layer) which is the primary element of a field effect transistor, silicon semiconductor compounds are most widely used. In general, a silicon single crystal is used in a high-frequency amplification element, an integrated circuit element or the like which require high-speed operation. On the other hand, in a liquid crystal driving element or the like, an amorphous silicon semiconductor (amorphous silicon) is used in order to meet the demand for an increase in area.
As examples of a TFT, an inverted-staggered TFT can be given in which a gate electrode, a gate-insulating layer, a semiconductor layer such as hydrogenated amorphous silicon (a-Si:H), source and drain electrodes are stacked on a substrate such as glass. This TFT are used, in a field of large-area devices including an image sensor, as a driving element for flat panel displays represented by active matrix-type liquid crystal displays. In these applications, with an improvement in function, an increase in operation speed is demanded even for conventional TFTs using amorphous silicon.
Today, as a switching element for driving a display, a device using a silicon-based semiconductor film constitutes the mainstream due to various excellent performances including improved stability and process ability of a silicon thin film and a high switching speed. Such a silicon-based thin film is generally produced by the chemical vapor deposition (CVD) method.
Meanwhile, a crystalline silicon-based thin film is required to be heated at a high temperature, for example, 800° C. or higher, for crystallization. Therefore, it is difficult to form a crystalline silicon-based thin film on a glass substrate or on a substrate formed of an organic substance. Accordingly, a crystalline silicon-based thin film can be formed only on an expensive substrate having a high thermal resistance such as silicon wafer and quartz. In addition, it has a problem that a large amount of energy and a large number of steps are required in production.
Further, since the application of a crystalline silicon-based thin film is normally restricted to a TFT with a top-gate configuration, a reduction in production cost such as a decrease in number of masks is difficult.
On the other hand, an amorphous silicon thin film, which can be formed at a relatively low temperature, has a lower switching speed as compared with a crystalline silicon semiconductor. Therefore, when used as a switching element for driving a display, a high-speed animation may not be displayed.
Further, when a semiconductor active layer is irradiated with visible rays, it exhibits conductivity, and current leakage occurs to cause malfunction, resulting in a deteriorated performance as a switching element. Therefore, a method is known to provide a light-shielding layer to shield visible rays. As the light-shielding layer, a thin metal film is known.
However, if a light-shielding layer formed of a thin metal film is provided, not only the production steps are increased but also a problem arises that, due to a floating potential, the light-shielding layer is required to be fixed to ground level, which results in generation of parasitic capacitance.
Specifically, in the case of a liquid crystal display television having a resolution of VGA, amorphous silicon having a mobility of 0.5 to 1 cm2/Vs could be used. For a liquid crystal display television having a resolution of SXGA, UXGA, QXGA or higher, a mobility of 2 cm2/Vs or higher is required. A further higher mobility is required if the driving frequency is increased in order to improve the image quality.
If amorphous silicon, of which the properties change by a DC stress, is used in an organic EL display which is driven by current, a problem occurred that image quality deteriorated if used for a long period of time.
In addition, if crystalline silicon is used in these applications, a demand for an increase in area could not be satisfied or the production cost increased since a high-temperature heat treatment was required.
Under such circumstances, in recent years, as a film which is more improved in stability than a silicon-based semiconductor thin film, an oxide semiconductor thin film using an oxide has attracted attention.
For example, Patent Document 1 discloses a TFT using zinc oxide as the semiconductor layer.
However, this semiconductor layer has a field effect mobility of as low as about 1 cm2/Vs·sec and a small on-off ratio. In addition, since current leakage tends to occur easily, practical application thereof on the industrial scale was difficult. Further, many studies have been made on an oxide semiconductor obtained by using zinc oxide which contains crystalline substances. If this oxide semiconductor is formed into a film by a sputtering method which is generally conducted on the industrial scale, the following problems occurred.
That is, a TFT may have deteriorated performance such as a low mobility, a small on-off ratio, a large amount of current leakage, unclear pinch-off and tendency of becoming normally-on. In addition, since this oxide semiconductor has poor chemicals resistance, the production process or the use environment had restrictions such as difficulty in wet etching. Further, in order to improve the performance, film formation was required to be conducted at a higher pressure, which caused industrial application to be difficult due to a lower film-forming speed and a higher treatment temperature exceeding 700° C. Further, TFT performance such as field mobility in a bottom-gate configuration was poor. In order to improve the performance, the film thickness was required to be 50 nm or more in a top-gate configuration, which restricted the TFT device structure.
In order to solve these problems, a TFT using an amorphous semiconductor film formed of indium oxide and zinc oxide has been studied (see Patent Document 2).
This oxide semiconductor film had a problem that a sufficient on-off ratio could not be obtained easily due to a high off current when used in a transistor.
As described in Patent Document 3, studies have been made on the application of a composite oxide containing indium elements, zinc elements and gallium elements, which oxide was conventionally studied as a transparent conductive film, to a TFT (see Non-Patent Document 1).
However, in a TFT using a semiconductor film composed of this composite oxide, in order to keep the S value small or to decrease a shift in threshold value by a stress, it was required to apply a substantial thermal history (for example, a heat treatment at a high temperature of 350° C. or higher for 1 hour or more or the like). The TFT is easily affected by surrounding environments such as light and air.    Patent Document 1: JP-A-2003-86808    Patent Document 2: U.S. 2005/0199959    Patent Document 3: JP-A-2000-44236    Non-Patent Document 1: Kim, Chang Jung et al. Highly Stable Ga2O3-In2O3-ZnO TFT for Active-Matrix Organic Light-Emitting Diode Display Application, Electron Devices Meetings, 2006. IEDM '06. International (ISBN: 1-4244-0439-8)
The invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide a field effect transistor having a high mobility and a small S value.
Another object of the invention is to provide the method for producing a field effect transistor which can attain improved properties by application of a thermal history at a low temperature or for a short period of time.